1. Field of the Invention
This invention relates to a process for the production of ethylene oxide. More particularly, it relates to a process for the production of ethylene oxide of high purity in high yields by introducing reactant ethylene and a molecular oxygen containing gas into a catalyst bed filled with a silver catalyst and subsequently causing the reaction product gas containing the formed ethylene oxide and issuing from the catalyst bed to be passed through a cooling zone filled with a carrier containing a metal capable of inhibiting isomerization of ethylene oxide to acetaldehyde.
2. Description of the Prior Arts
In processes for the production of ethylene oxide by the vapor-phase catalytic oxidation of ethylene with molecular oxygen, a major concern of those engaging in the art resides in the problem as to how ethylene oxide is produced in high yields with high productivity and at high purity. In search of a solution to this problem, numerous studies have been carried out and various efforts made to date. They have been preponderantly directed to improvement of catalysts and improvement of reaction conditions. Of the various subjects of studies, the improvement of catalysts has constituted itself the most important problem. Numerous efforts have been focussed on the improvement of catalysts' qualities.
As is widely known, in the production of ethylene oxide by the vapor-phase catalytic oxidation of ethylene with molecular oxygen in the presence of silver catalyst, the reaction proceeds as represented by formula I shown below. ##STR1##
This reaction, however, is susceptible of simultaneously entailing side reaction, which are represented by a so-called perfect oxidizing reaction of a mechanism as expressed below by formula II. EQU H.sub.2 C.dbd.CH.sub.2 +30.sub.2 .fwdarw.2H.sub.2 O+2CO.sub.2 (II)
The extent to which the reaction of formula II is suffered to occur depends on the quality of the catalyst in use. This explains why the improvement of catalysts has constituted one major task for the researches.
In addition to the reactions of formula I and formula II indicated above, there is another reaction which affects the yield of ethylene oxide. It is isomerization of ethylene oxide to acetaldehyde which proceeds as represented by formula III below. ##STR2##
No matter how high the selectivity of the conversion of ethylene to ethylene oxide may be, the yield of ethylene oxide is lowered when the ratio of isomerization of the formed ethylene oxide to acetaldehyde is increased. An addition to the acetaldehyde content of the product stream results in a proportional increase in the expense incurred in the refining of the product stream for the isolation of ethylene oxide at high purity. In this respect, therefore, minimization of the isomerization of ethylene oxide to acetaldehyde poses itself as one of the important tasks in the way of the production of ethylene oxide.
Generally in processes for partial oxidation of hydrocarbons, it is known to preclude possible occurrence of side reactions by passing the reaction product gas through a cooling zone filled with packing (U.S. Pat. No. 3,147,084 to E. P. Franzen et al). It is also known to apply such a method to the processes for the production of ethylene oxide by the vapor-phase catalytic oxidation of ethylene with molecular oxygen (British Pat. No. 1,449,091 to Snamprogetti and U.S. Pat. No. 4,061,659 to Nielsen et al). The British patent, for example, discloses a method resorting to passage of the reaction product from a reaction zone through a cooling zone being contiguous or not contiguous to a reaction zone and containing or not containing bodies of inert materials. The U.S. patent to Nielsen et al teaches a method resorting to passage of the reaction product through a cooling zone filled with an inert refractory particulate material having a surface area of 0.1 m.sup.2 /g or less. These methods, however, are not sufficiently effective in providing satisfactory repression of the isomerization of ethylene oxide to acetaldehyde.
It has been ascertained by our studies that the isomerization of ethylene oxide to acetaldehyde has bearings upon the magnitude of the temperature of the reaction product gas containing the formed ethylene oxide, the residence time of the gas at elevated temperatures and the surface area in which the gas is exposed to contact with the articles of elevated temperatures. Repression of the isomerization is advantageously accomplished, therefore, by a method capable of lowering the temperature of the ethylene oxide containing gas, a method of shortening the residence time at elevated temperatures or a method of decreasing the surface area of contact with the articles of elevated temperatures.
In ordinary conventional processes for the production of ethylene oxide, the hot reaction product containing the formed ethylene oxide and issuing from the reactor is led to an ethylene oxide absorption tower, wherein it is recovered by absorption in water kept at a temperature below 35.degree. C. Generally, prior to delivery to the absorption tower, the gas is cooled down to a proximity of room temperature by means of a varying heat exchanger. In the interval between the outlet of the catalyst bed and the inlet of the heat exchanger, however, the reaction product gas containing the formed ethylene oxide is left to cool spontaneously or subjected partially to forced cooling by means of a coolant and, consequently, is suffered to remain at a temperatuure above 200.degree. C. This retention of the gas at the high temperature forms a main cause for the isomerization of ethylene oxide to acetaldehyde. It would appear to be effective, therefore, to cool the reaction product gas suddenly to a temperature below 200.degree. C. in this interval. On account of the expense required for the particular step of sudden cooling and in the light of the economics of commercial operation of the whole process, it is not necessarily advantageous to relay solely on the step of sudden cooling for the repression of isomerization. In fact, it is difficult to obtain thorough repression of the isomerization merely by this step of sudden cooling.
As more advantageous means of effectively repressing the isomerization of ethylene oxide to acetaldehyder, it may be necessary to devise a method for shortening the residence time of the gas at elevated temperatures and providing efficient cooling of the gas. For this purpose, there is apparent necessity of using a packing carried in a cooling zone which is formed in the interval between the catalyst bed and the heat exchanger where the gas has a temperature of not lower than 150.degree. C. As the packing, it is desirable from the physical and economic points of view to use an inert refractory particulate material of the kind used for the production of a silver catalyst. Of course, other kinds of packings may be used for this purpose on condition that due consideration be paid to factors such as mechanical strength, weight, pressure loss and economics.
One important consideration required at this point, however, is that the act of filling the empty space with a packing results in an addition to the area of contact of the reaction product gas, which in turn accelerates the isomerization of ethylene oxide in utter contradiction to what is aimed at by the packing. Most packings are such that at temperatures required for the formation of ethylene oxide, they accelerate the isomerization of ethylene oxide to acetaldehyde in the surface of contact thereof with the reaction product gas containing the formed ethylene oxide. The inert refractory particulate material previously described as forming a substance desirable for the production of silver catalyst is no exception. On the contrary, the inert refractory particulate material, when given a large specific surface area, may well be regarded as contributing more to accelerating the isomerization than any other material.
An object of the present invention, therefore, is to provide a process for the production of ethylene oxide of high purity in high yields by repressing the isomerization to acetaldehyde of the ethylene oxide formed by the vapor-phase catalytic oxidation of ethylene with molecular oxygen.
Another object of the invention is to provide a process for the production of ethylene oxide in high yields by increasing the oxygen concentration in the influent mixed gas used for the production of ethylene oxide.